In relation to a portable radio equipped with such an antenna (e.g., a cellular phone), there is recently a growing demand for addition of functions of a plurality of radio systems; for instance, a GPS and Bluetooth (Registered Trademark). When an attempt is made to provide a cellular phone with a plurality of radio systems, a range of working frequency band becomes broader. For instance, the portable radio must be made compatible with an 800 MHz band, a 1.7 GHz band, and a 2 GHz band for communication of a cellular phone. Specifically, the portable radio must be made compatible with a 1.5 GHz band for GPS and 2.4 GHz band for Bluetooth. Accordingly, when an attempt is made to equip the cellular phone with such plural radio systems, a built-in antenna must ensure predetermined antenna performance for a plurality of frequency bands.
A rectangular-parallelepiped-shaped antenna element 200 has hitherto been proposed as shown in FIG. 7 (see; for instance, Patent Document 1). In the antenna 200, a rectangular-parallelepiped-shaped antenna element 201 whose minimum side is smaller than λ/8 (λ: a wavelength) is connected to a coaxial cable 202 and disposed in close proximity to a ground plate 202. It is shown that use of the rectangular-parallelepiped-shaped antenna element 202 makes a bandwidth broader.
An antenna element described in connection with Patent Document 2 shown in FIG. 8 has already been known as such a rectangular-parallelepiped-shaped antenna element. In an antenna element 300 described in connection with; for instance, Patent Document 2, a conductor plate 301 is connected to a conductor ground plate 303 by way of a metal wire 302 as shown in (A) of FIG. 8, and power is fed from a feeding point 305 by way of a metal wire 304. Meanwhile, a conductor wall 306 is electrically connected at the other end to an electromagnetic coupling adjustment plate 307, as well as being electrically connected at one end to the conductor plate 301. The electromagnetic coupling adjustment plate 307 is disposed while spaced at a predetermined gap away from the conductor ground plate 303 as shown in (B) of FIG. 8, thereby forming a capacitor between the conductor ground plate 303 and the electromagnetic coupling adjustment plate 307.
Incidentally, the antenna element 300 makes a frequency low by arranging the conductor wall 306 and the electromagnetic coupling adjustment plate 307; for instance, in such a way that a path from a shortcircuit area where the metal wire 302 is connected to the conductor plate 301 to an open end of the electromagnetic coupling adjustment plate 307 becomes longer. In particular, an arrangement is made in such a way that a current path from a feeding point where the metal wire 304 is connected to the conductor plate 301 to the shortcircuit area comes to a half wavelength of a desired resonance frequency, whereby both a reduced resonance frequency of an antenna and a broader band of a frequency characteristic are accomplished.    Patent Document 1: JP-A-2006-279159    Patent Document 2: JP-A-2002-223114